The present invention relates in general to a ride-through performance of induction motors. Three phase induction motors are rotor based electric motors having a fixed stator that is positioned about the perimeter of the rotor.
To operate the induction motor, portions of the stator field are consecutively energized thereby inducing an electromagnetic field about the rotor which field rotates at a slower speed than the rotating magnetic field of the stator. Three phase induction motors are typically used to drive rotating equipment, however, induction motors are sensitive to power fluctuations. Power fluctuations or momentary power interruptions are a momentary loss of power quickly followed by a restoration of power which may inturn result in damage to the motor. Causes of such power fluctuations may be due to weather effects on the power transmission line equipment, from buss transfers clearing other faults on transmission lines, from switching of generation sources at the power utility, or from internal buss transfers such as the switching to backup generator systems. These momentary power interruptions cause insufficient power flow or lack of power flow to the induction motor. During a momentary loss of power, the induction motor may act as a generator while the rotor remains rotating. If the power is re-established to the induction motor while the rotor is still rotating, damage may result to the induction motor in the form of stress and negative torque as a result of the phase differential between the line voltage and the motor generated back emf generated voltage. The stator end windings and the rotor shorting rings are exposed to excessive stress. This excessive stress, although not a threat of immediate failure, often emerges as the cause of accelerated motor wear and reduced motor life.
The most significant damage from momentary power outages is the direct result of large transient torques. Transient torques up to twelve times a full load torque (or twenty times a full load torque if power correction capacitors are used) can be experienced as the result of a momentary power outage. The large transient torques that result from the momentary power outage may be negative at times at times thereby attempting to reverse the direction of rotation of the motor and driven equipment. Such large transient torques which typically exceed the structural design limitations of a motor and associated equipment, can cause immediate catastrophic failure.
Common methods currently used to prevent damage to the motor and associated equipment during a momentary power outage disconnect the motor from the supply power and prevent the reconnection of the power supply until the motor has stopped. After the induction motor is de-energized and the rotor has stopped rotating, power is then reconnected to re-energize the induction motor. This typically requires that all equipment operating from a common control unit or power source is shut down until all induction motors have ceased rotational operation so that damage does not occur to the motor and the equipment driven by the motor. However, manufacturing facilities utilize equipment using induction motors which are required to continuously operate to maintain production rates. Typically, if a portion of the plant or process is shutdown to prevent damage to the induction motor and associated equipment, the entire manufacturing facility must be shutdown in an orderly manner to prevent any damage the respective equipment. Since a majority of momentary power outages are caused by transmission problems, often equipment in all or major portions of the plant or process are affected simultaneously. This is a time consuming and inefficient operation for a facility to have to completely shut down a portion or all of its operation each time a power fluctuation or power loss is present.
Another method of controlling damage to the induction motor and associated equipment during times of momentary power outages is to use an uninterruptible power supply that provides power from a battery and inverter during the momentary power outage. This basically eliminates the power outage as seen by the equipment. However, such uninterruptible power supplies are typically used with very small motors due to the problems of having to supply large amounts of power from one or more batteries while maintaining a respective state of charge for powering the equipment. Due to the relatively large cost, the use of uninterruptible power supplies to provide the large amounts of power as required by the equipment is not considered practical for most three phase induction motor applications.